The burden of non-specific low back pain in the population is substantial. Putative sacroiliac joint pain or 'sacroiliac syndrome' is a significant subset of this condition. Localisable pain over the long posterior sacroiliac ligament (LPSL) is often described as both a clinical sign and symptom of this syndrome. The aim of this qualitative morphological investigation was to investigate a potential basis for a patho-anatomic relationship between the long posterior sacroiliac ligament (LPSL) and the posterior sacrococcygeal plexus (PSP). Twenty-two cadavers were available for study. Three methods were utilised: macro-dissection (n = 22 sides), small wax block histology (n = 3 sides), large wax block histology (n = 1 side), giant wax block histology (n = 4 sides) and a review of E12 sheet plastinated transverse and sagittal sections (n = 4 sides). The LPSL was demonstrated to have a layered structure that was penetrated by the lateral branches of the dorsal sacral rami. A segmental relationship was observed between the lateral branches of the dorsal sacral rami and the LPSL. The posterior layer of the thoracolumbar fascia was not continuous with the superficial layer of the LPSL, deep to the fibres of gluteus maximus. The LPSL was shown to have three morphologically distinct regions, the proximal, mid and distal LPSL. A confluence of three layers was observed at the mid LPSL posterior to the sacroiliac joint (SIJ) namely the erectores spinae aponeurosis (ESA), the gluteal aponeurosis (GA) and the deep fascial layer from the second and third medial dorsal sacral foramina. A region of adipose and loose connective tissue was evident deep to the deep fascial layer in which lateral branches of the dorsal sacral rami were observed. In the region of the mid LPSL between the inferior PSIS and the third sacral transverse tubercle (ST), the attachment of the ESA to sacral bone was absent. This space was occupied by the continuous underlying region of adipose and loose connective tissue. At its greatest extent, this region was observed between the central sacral canal medially and the gluteal aponeurosis laterally. This study suggests that the LPSL may be better described as a retinaculum for the lateral branches of the dorsal sacral rami. These morphological findings provide a basis for the existence of a potential patho-anatomical mechanism that may explain localised pain in the posterior sacroiliac region, usually interpreted as referred pain from the sacroiliac joint. The lateral branches of the dorsal sacral rami may have a potential vulnerability to trauma or ischaemic challenge in this region, which may account for SIJ related 'non-specific' low back pain or for pregnancy related peripartum pelvic pain. Furthermore, a morphological basis for the anatomical confounding of SIJ tests is shown.